In total, at least 57 people were killed in the assault, which came a day after army members defected to the opposition in the area.

The Syrian Observatory for Human Rights said Thursday that the figure includes six children and seven women.

"The defectors took refuge in the area of Al-Sanamein and Ghabagheb, which up until then had remained somewhat more calm than other areas of Daraa," Observatory spokesperson Rami Abdel Rahman told AFP.

In the ambush, 16 rebels were also killed, the Observatory reported, as were 12 regular soldiers, and three army defectors, in a combination of shelling, fighting and summary executions.

Several houses were destroyed in the bombardment, the Observatory reported, and residents were detained.

The crucial southern border area – where the uprising initially began over two years ago – has become an increasingly key battleground for rebels over recent weeks.

Control of the area would help secure the road to Damascus, and control of the capital, where President Bashar Assad’s most loyal troops and supporters reside, reaching which is arguably the rebels’ end goal.

"We are always considering a variety of options, we are going to continue to aid the opposition, working with them in terms of what they need, in terms of what we're willing to provide," a top State Department official said.

There are concerns over a proliferation of weapons in Syria, particularly among certain opposition groups, such as the Nusra Front, which the U.S. last year blacklisted as a terrorist organization.

Al-Qaeda in Iraq this week said that it had merged with the Nusra Front, a statement the Syrian group appeared to distance itself from Wednesday. However, Abu Mohammad al-Golani, the leader of the Nusra Front, also pledged allegiance to Al-Qaeda chief Ayman al-Zawahri.